Mixed-valence Cobalt Complex Research Articles

Synthesis, structure and catecholase activity of a trinuclear mixed valence cobalt complex

A trinuclear mixed valence cobalt complex, [L3CoIICoIII2(CH3COO)(OCH3)] (2), has been synthesized using a Schiff base ligand (H2L, 1). The ligand was prepared via 1:1 condensation of 3-Chloro salicylaldehyde and 3-Amino-1-propanol in methanol. The structure of 2 has been elucidated by single crystal X-ray crystallography. Comparing each set of the corresponding data of bond lengths and bond angles, presence of two different oxidation states (+2 and +3) of the cobalt metal ions was understood. Again the occurrence of mixed valence has been confirmed by X-ray photoelectron spectroscopy (XPS) study of 2. Interestingly, 2 acted as an efficient bio-mimic of catechol oxidase toward oxidation of 3,5-di‑tert-butyl catechol (3,5-DTBC) to 3,5-di‑tert-butyl quinone (3,5-DTBQ). Two different solvents, methanol (CH3OH) and acetonitrile (CH3CN), have been utilized as media for exploring catecholase activity of 2 and acetonitrile was observed to be a better medium since furnished greater turn-over number. 1HNMR spectroscopy suggests completion of the oxidation process within just ten minutes in acetonitrile. The ESI-MS spectrum of the substrate-catalyst mixtures in acetonitrile confirmed formation of Co(III)-3,5-DTSQ (3,5-DTSQ = 3,5-di‑tert butyl semiquinone) intermediates in the time scale of mass spectrum which could stand as a supporting evidence for the catalytic function of 2. However, all the substrate bound intermediates observed are mononuclear which indicates that the complex probably undergoes a dissociation in acetonitrile to generate mononuclear fragments for binding the substrate. Thus, on the basis of ESI-MS, 2 may be suggested as a pre-catalyst.

Exploration of noncovalent interactions in the solid state structures of carboxylate bridged trinuclear mixed valence cobalt complexes using computational tools based on the topological analysis of the electron density

Three linear mixed valence trinuclear cobalt(III)-cobalt(II)-cobalt(III) complexes, [CoII{(μ-L1)(μ-OOCCH3)CoIII(OOCCH3)}2]⋅4H2O (1), [CoII{(μ-L2)(μ-OOCCH3)CoIII(OOCCH3)}2]⋅4H2O (2) and [CoII{(μ-L1)(μ-OOCPh)CoIII(OOCPh)}2] (3), have been synthesized using two N2O2 donor 'reduced Schiff base' ligands, H2L1 (2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(4-chlorophenol) and H2L2 (2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(4-bromophenol), and acetate or bezoate as anionic co-ligands. The complexes have been characterized by spectroscopic measurements and their solid state structures have been determined by single crystal X-ray diffraction analysis. DFT calculations have been performed to study the mixed valence nature of the complexes. Moreover, the structure directing role of several noncovalent interactions in the solid state of the complexes has been investigated. The interactions have been characterized by a combination of two computational tools based on the topological analysis of the electron density, which are the quantum theory of atoms-in-molecules (QTAIM) and the noncovalent interaction plot, since they are useful to reveal noncovalent interactions in the solid state.

A Highly Active Nitrogen‐Doped Mixed‐Phase Mixed‐Valence Cobalt Nanocatalyst for Olefins and Nitroarenes Hydrogenation

AbstractDevelopment of an inexpensive, environmentally friendly and robust recyclable heterogeneous catalyst for alkene and nitroarene hydrogenation is in high demand for the sustainable future of chemical industries. Herein, we have synthesize and characterized a trinuclear mixed valence cobalt complex 1 and nitrogen doped mixed phase cobalt (β‐Co(OH)2 and CoO) nanocatalyst (N@MPCoNC) as reported earlier. N@MPCoNC was synthesized via single source molecular precursor by reducing 1 by aq. N2H4 followed by calcinations. Both mixed‐valence cobalt metal complex (1) and nitrogen doped mixed phase cobalt nanocatalyst (N@MPCoNC) were utilized for alkenes and nitroarenes hydrogenation reaction, however excellent catalytic activity was noticed by N@MPCoNC with the 100 % conversion of styrene and 4‐nitrotoluene to the corresponding ethylbenzene and 4‐aminotoluene with 100 % selectivity under mild reaction condition. The scope of the heterogeneous N@MPCoNC was further evaluated for other alkenes and nitroarenes to obtain corresponding alkanes and aminoarenes with an excellent conversion and high selectivity at room temperature.

One-pot synthesis of cobalt complexes with 2,6-bis(arylimino)phenoxyl/phenthioxyl ligands and catalysis on isoprene polymerization.

Several cobalt complexes supported by 2,6-bis(arylimino)phenoxyl/phenthioxyl ligands κ2N,X-Ar[NXN]CoCl(THF) (1a, X = O, Ar = 2,6-Me2C6H3; 1b, X = O, Ar = 2,6-iPr2C6H3; 2a, X = S, Ar = 2,6-Me2C6H3; 2b, X = S, Ar = 2,6-iPr2C6H3) were synthesized by direct oxygen(sulfur) insertion into the C-Co bond of the mixed-valence cobalt complexes {κ2C,N,η6-Ar[NCN]Co-κN-CoCl(μ-Cl)}2. Crystallization of 1b in the presence of water gave the hydrolysis product 1b'. Treatment of Ar[NCN]Li with dioxygen followed by the addition of CoCl2 afforded the heteroatomic complexes {κ2N,O-Ar[NON]Co(μ-Cl)2Li}2 (3a, Ar = 2,6-Me2C6H3; 3b, Ar = 2,6-iPr2C6H3) or κ2N,O-Ar[NON]Co2Cl2(μ-Cl)2Li(THF)2 (4a, Ar = 2,6-Me2C6H3; 4b, Ar = 2,6-iPr2C6H3) depending on the amount of CoCl2 used. The Co(iii)/Li heterometallic complex 3b' with imino-phenoxyl-amino ligands was formed probably via a redox reaction of 3b. The reactions of Ar[NCN]Li with elemental sulfur and CoCl2 gave κ2N,S-Ar[NSN]Co2Cl2(μ-Cl)2Li(THF)2 (5a, Ar = 2,6-Me2C6H3; 5b, Ar = 2,6-iPr2C6H3) respectively. These complexes were well characterized by FT-IR and elemental analyses, and the molecular structures of 1b', 3b', 4a, and 4b were confirmed by X-ray crystallography. Upon activation with Al2Et3Cl3 in toluene, these complexes showed high activities in isoprene polymerization affording cis-1,4 enriched polymers with a moderate molecular weight (0.85-4.72 × 104 Da).

Field-induced slow magnetic relaxation from linear trinuclear CoIII-CoII-CoIII to grid [2 × 2] tetranuclear mixed-valence cobalt complexes.

By employing the ligand azotetrazolyl-2,7-dihydroxynaphthalene (H3ATD), two linear trinuclear mixed-valence cobalt complexes [CoIICoIII2(HATD)4(H2O)4]·4DMA·3H2O (1, DMA = N,N-dimethylacetamide) and [CoIICoIII2(HATD)4(DMF)2(H2O)2]·2DMF·2H2O (2, DMF = N,N-dimethylformamide) were synthesized. Two [2 × 2] grid-like tetranuclear ion-pair complexes [CoII2CoIII2(HATD)4(bpp)2(H2O)2][CoIII(HATD)2]2·8DMF·6H2O (3, bpp = 2,6-di(pyrazol-1-yl)pyridine) and [CoII2CoIII2(HATD)4(bpp)2(H2O)2][CoIII(HATD)2]2·8DMSO·4MeOH (4, DMSO = dimethyl sulphoxide) were obtained by the reaction of complex 1/2 with tridentate-chelating bpp in DMF and DMSO, respectively. The single-crystal X-ray diffraction analysis indicated that complexes 1 and 2 have a similar core, in which the DMA in 1 acts as a guest molecule, and the DMF in 2 acts as a coordinated molecule and guest molecule. Complexes 3 and 4 are isostructural. All the Co(ii) ions in 1-4 are present in a distorted octahedral geometry. The ac susceptibility measurements show that all complexes display frequency-dependent peaks in the out-of-phase (χm'') component of the alternating-current (ac) magnetic susceptibility data, which is the characteristic behavior of single molecule magnets (SMMs).

Magneto-structural studies of two M–O–M bridged homochiral mixed valence Co(II)/Co(III) complexes

Two mixed-valence cobalt complexes, [CoIICoIII2(mpm)6](ClO4)2 (1) and [CoII2CoIII2(μ3-OMe)2(μ-mpm)4(NO3)4] (2), where mpm is the deprotonated form of R or S α-methyl-2-pyridinemethanol, have been structurally and magnetically characterized. Both complexes crystallize in chiral space groups, where 1 is a trinuclear compound with a linear Co(III)–Co(II)–Co(III) topology and 2 is a tetranuclear cluster with a defect dicubane-type structure that has two missing vertices. In both cases the mpm− ligand bridges adjacent Co ions in a η1:η2:μ manner. The molecular geometries of the complexes reveal the Co ions are octahedral, where the greatest deviation from Oh symmetry is apparent for the HS d7 Co(II) ions. The chirality of 1 and 2 were studied by circular dichroism spectroscopy where opposite Cotton effects are observed for the R- and S-enantiomers of both complexes. Magnetic susceptibility studies reveal the presence of largely quenched first order orbital angular momentum together with weak ion or intermolecular dipole exchange.

A mixed-valence complex of cobalt based on 3-methoxysalicylaldoxime

A new tetranuclear mixed-valence cobalt complex, namely di-μ2-azido-diazidodiethanolbis{μ2-2-[(hydroxyimino)methyl]-6-methoxyphenolato}bis{μ3-6-methoxy-2-[(oxidoimino)methyl]phenolato}dicobalt(II)dicobalt(III) ethanol disolvate, [Co(II)2Co(III)2(C8H7NO3)2(C8H8NO3)2(N3)4(C2H5OH)2]·2C2H5OH, has been synthesized by the reaction of Co(OAc)2·4H2O (OAc is acetate) with 3-methoxysalicylaldoxime (H2mosao) in an ethanol solution. In the complex, the four Co cations all display distorted octahedral coordination environments and they are bridged by two κ(2),κ(1),κ(1);μ3-mosao(2-) ligands, two κ(2),κ(2);μ2-Hmosao(-) ligands and two μ2-N3(-) anions to form a tetranuclear [Co4N4O4] cluster. Adjacent clusters are connected through weak C-H···N and C-H···O interactions, resulting in a two-dimensional supramolecular network parallel to the ac plane. The magnetic properties of the complex have also been studied.

Iso-valence Co(II) and mixed-valence Co(II/III) tetranuclear complexes: Synthesis, structure, magnetic properties and DFT study

The reaction of Co(ClO4)2·6H2O with symmetrical N4O3 coordinating heptadentate ligand (H3L) in presence of triethylamine yielded cobalt(II) tetranuclear complexes [Co4(L)2](ClO4)2, 1. When the same reaction is carried out with sodium azide instead of triethylamine afforded tetranuclear mixed-valence Co(II/III) species, [Co4(L)2(μ1,3–N3)2](ClO4)2, 2. Here L3− is the deprotonated form of 2,6-bis[{{(1-hydroxy-2-methylpropan-2-yl)(pyridine-2-ylmethyl)}amino}methyl]-4-methylphenol. Complex 1 crystallizes in the cubic system, space group Fd-3c, with unit-cell parameters a=b=c=43.320(5)Å, Z=48, whereas complex 2 crystallizes in the space group I41/a in the tetragonal crystal system with Z=8 and unit-cell parameters a=b=16.3243(15)Å, c=50.093(4)Å. The X-ray structures of 1 and 2 have revealed a CoII4O6 core in 1 and a [CoII2CoIII2O2(μ1,3–N3)2] core in 2. Tetranuclear Co(II) compound shows interesting magnetic property and exhibit antiferromagnetic interaction between the paramagnetic centers. Density functional theory has been used to justify the exchange pathways for 1. Exchange coupling in the mixed valence cobalt complexes studied is insignificant, due to the large distances of separation of the Co(II) centers in tetranuclear units.

Syntheses, crystal structures and magnetic properties of mixed-valence linear trinuclear cobalt and 2D nickel complexes

Abstract Two new complexes, [CoL2]2[Co(CH3OH)4] (1), and [Ni2L2(CH3OH)2]n (2) (H2L = 2-(((2-hydroxynaphthalen-1-yl)methylene)amino)acetic acid), have been synthesized and structurally characterized. Single-crystal X-ray analysis reveals that complex 1 is linear trinuclear mixed valence cobalt complex, while complex 2 is a 2D complex by double naphtholate bridged and syn–anti carboxylate bridged. Magnetic susceptibility measurements indicate that 1 shows mononuclear CoII behavior whereas 2 exhibits an antiferromagnetism behavior.

Structure of a catalytically active octanuclear cobalt complex [Co(II)4Co(III)4(μ4-O)4(μ-O2CPh)12(PhCOOH)4

The structure of a catalytically active octanuclear mixed-valence cobalt complex [Co(II)4Co(III)4(μ4-O)4(μ-O2CPh)12(PhCOOH)4] (Ph = phenyl, C6H5) has been determined using an X-ray diffraction analysis. The presence of 16 ligands containing phenyl rings in each complex gives rise to strong steric stresses and leads to a disordering of terminal groups. The compound crystallizes in the monoclinic crystal system, space group of symmetry P21/n, with the unit cell parameters a = 18.076(5) A, b = 26.371(3) A, c = 22.791(5) A β = 92.48(1)°, V = 10853(4) A3, Z = 4, and ρ = 1.516 g/cm3. The final values of the discrepancy factors are R1 = 0.0441 for 4005 symmetrically nonequivalent reflections with I ≥ 2σ(I), wR2 = 0.1131, and GooF = 0.943.

Structure of a catalytically active octanuclear cobalt complex [Co(II)4Co(III)4(μ4-O)4(μ-O2CPh)12(C2H5OH)4] · 3C3H6O · C2H5OH

The structure of a catalytically active octanuclear mixed-valence cobalt complex [Co(II)4Co(III)4(μ4-O)4(μ-O2CPh)12(C2H5OH)4] · 3C3H6O · C2H5OH (Ph = phenyl, C6H5), which is characterized by the presence of chelate open cavities, has been determined using X-ray diffraction. The compound crystallizes in the triclinic crystal system, space group of symmetry P $\bar 1$ , with the unit cell parameters a = 14.385(5) A, b = 15.198(5) A, c = 25.448(9) A, α = 91.888(4)°, β = 95.641(4)°, γ = 102.750(4)°, V = 5391(3) A3, Z = 2, and ρ = 1.474 g/cm3. The final values of the reliability factor are R 1 = 0.0489 for 11164 nonequivalent reflections with I ≥ 2σ(I), wR 2 = 0.1472, and GoofF= 1.003.

Iso- and mixed-valent phenoxy bridged binuclear macrocyclic complexes of cobalt, iron and manganese

The binuclear complexes of a tetraaminodiphenol macrocyclic ligand (H 2L) of cobalt(II) and manganese(II) with the composition [M 2LX 2]βH 2O (X = Cl or Br) and of iron(II) with the composition [Fe 2L(imidazole) 2(CH 3OH) 2](ClO 4) 2·2CH 3OH have been synthesized and spectroscopically characterized. Cyclic voltammetric measurements of the dicobalt(II) complexes have indicated that the generation of Co III-Co II and Co III-Co III species take place quasi-reversibly or irreversibly at about 0.38 and 0.90 V vs SCE, respectively. The dimanganese(II) complexes undergo quasi-reversible one-electron oxidation at about 0.32 V, but further oxidation does not occur up to 1.2 V. The iron(II) complex irreversibly gets oxidized at ca 1 V. The dicobalt(II) and dimanganese(II) complexes can be oxidized with bromine and the mixed-valence complexes [M IIIM IILCl 2Br(CH 3OH)]·solvent have been isolated. Variable-temperature magnetic measurements of [Co IIICo IILCl 2Br (CH 3OH)]·0.5CH 2Cl 2, [Mn 2 IILBr 2(CH 3OH) 2]·H 2O and [Fe 2 IIL(imidazole) 2(CH 3OH) 2] (ClO 4) 2·2CH 3OH have indicated that the mixed-valence cobalt complex behaves as a S = 3 2 system, while both the diiron(II) and dimanganese(II) complexes exhibit antiferromagnetic exchange coupling with J = −6.3 and −6.1 cm −1, respectively.